1. Field of the Invention
This invention relates to apparatus for conveying a gaseous stream containing particulate matter and more particularly to conveying such a stream in conjunction with systems for the gasification of carbonaceous materials.
2. Description of the Prior Art
Process systems for gasifying solid carbonaceous materials, such as coal, typically involve conveying particulate material entrained in gaseous streams at a wide range of temperatures and pressures. Temperatures can range anywhere from ambient up to 1800.degree. F. or higher. Pressures can range from atmospheric up to 500 lbs./square inch or higher. In a typical fluidized bed coal gasification system, particles of coal are conveyed from a feed source, such as a coal lockhopper, and injected into a gasifier vessel. The typical method of conveyance is by entrainment of the particles in a gaseous process medium, such as a recycled product gas. The product gas discharged from the gasification vessel may typically comprise a mixture of combustible product gas, ungasified coal particles, partially gasified coal particles called char, or liquid globules of molten ash, which typically is made up of one or more compounds of any of Fe.sub.1-x S, Fe, Na, Al, K or SiO.sub.x. These different particles and globules are generally collectively called fines. In order to use the combustible product gas, it is necessary to separate these entrained fines from the gas, and this is typically done through the use of a centrifugal separator. However, since the fines may contain residual amounts of carbon, recycle of these fines back into the gasifier vessel would be beneficial. Further, since these fines already contain significant residual heat, it would be advantageous to recycle them at as high a temperature as is possible.
Previously existing systems have generally conveyed such fines in pipes made of materials such as carbon steel or refractory ceramic. These materials, however, have poor characteristics of erosion, tensile strength, flexibility, insulation or a combination of several. Materials which have very good erosion resistance typically will have very poor fabrication properties, such as poor bending qualities or high brittleness.
In general, solids conveyancing means of the prior art are directed towards straight-line flow problems and any interior lining is generally of a single material. Any such liner will represent a tradeoff between a material with favorable pipe fabrication qualities and a material with favorable erosion resistance qualities. Further, because erosion resistance varies with the radius of curvature, what may have satisfactory wear characteristics for straight pipes might have to be replaced in the event that a curve or flow perturbations or other problems cause the single material to wear out.
What is needed is an apparatus which will allow a change in direction of flowing particulate solids, have the ability to adapt to flow perturbations, have satisfactory erosion, tensile strength and flexibility characteristics, and be suitable for use at varying temperatures without substantial loss of heat.